Method and apparatus for comminuting or drying materials



Jan. 15; 1946.

N. N. STEPHANOFF METHOD AND APPARATUS FOR COMMINUTINC OR DRYINGMATERIALS Filed Feb. 28, 1940 3 Sheets-Sheet l piazza f =EVJ.

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N. N." STEPHANOFF I METHOD AND APPARATUS FOR COMMINUTING OR DRYINGMATERIALS Filed Feb. 28, 1940 '3 Sheets-Sheet 2 Cr v Arm/f era.

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METHOD AND APPARATUS FOR COMMINUT- IN G R DRYING MATERIALS Nicholas N.Stephanofl', Bryn Mawr. Pa., assignor to Thermo-Plastics Corporation,Camden, N. J

a corporation or New Jersey 7 Application February 28, 1940, Serial No.321,328

'(Cl. 24l5)- 10 Claims.

This invention relates to a'method and p ratus for commuting and/ordrying materials, these terms being used in a broad sense to include thebreaking up of large particles, the grinding of particles, and thedrying of material in the form of droplets or particles and theproduction of solid particles by cooling a molten liquid or by reactionwith, or of, a liquid, e. g., polymerization, and more particularlyrelates to a method and apparatus for effecting such results by theaction of high velocity gas or vapor jets. This application is in part acontinuation. of my application Serial No. 145,421, filed May 29, 1937.

While reference is made herein particularly to grinding or comminutionof solid and dry materials, it will be understood that the invention isequally applicable .to drying in a broad sense, as will be evident fromconsideration of the disclosure of my application Serial No. 199,687,filed April 2, 1938. It will also be evident that there may be adaptedto the present invention the principles set forth in my applicationSerial No. 235,139, filed October 15, 1938.

The present apparatus is concerned primarily with the utilization ofstaging for securing the most emcient use of the heat and pressureenergy of the elastic fluid used for the production of high velocityjets. In the production of grinding or the breaking up of droplets toeffect the most efficient drying, it i essential ,for best results,

including most eifective entrainment of the material in jets, to produceas much turbulence as possible in the jets into which the material to becomminuted or dried is introduced. While from the standpoint of securingmaximum kinetic energy of gases from a nozzle the best thermalefllciency is obtained when the fluid is fully expanded in aconverging-diverging nozzle, the fiow from such nozzles is relativelynon-turbulent as compared with the flow from nozzles of an abrupt type.These latter nozzles produce sufiicient eddies and whirls for entrappingthe'material and increasing the frequency of impacts of particles oneach other.

If at least a critical pressure drop is provided through a nozzle, a.high velocity which will be the acoustic velocity for the elastic fluidat the temperature and pressure conditions xisting in the jet will besecured. If 'a greater drop is provided and the nozzle suitably formedto take advantage of complete expansion with such drop, highervelocities are secured, but the increase in velocity is not efiective toproduce any corresponding increase in the extent of grinding.-

In accordance with the present invention, an

arrangement is provided for expanding the fluid efliciently in stages tothe lowest practical final pressure, producing at the same time apressure drop in each stage to a final pressure only slightly below thecritical pressure corresponding to that at the entrance of the nozzle,thereby obtaining very high jet velocities in each stage and producing,since abrupt or over-expanded nozzles can be used in at least some orthe stages, the most eflective turbulence for impact grinding ofparticles. For this purpose there are used several chambers, thematerial passing from one chamber into the other with successiveexpansion of the fluid in the stages to the final exhaust pressure.Successive stages of the type just indicated require, of course, the useof chambers which are sealed. to maintain the fluid under the requiredpressures for delivery to successive stages.

In accordance with the present invention, to

secure the maximum eificiency, there may be provided jacketing for thechambers in which the grinding takes place and also for reheating of theelastic fluid between the stages, preferably by the introduction ofsmall amounts of high temperature and high pressure fluid whichadditionally serves to increase the velocity of flow.

It is a further object of the invention to pro vide a selective actionwhereby large particles are retained in a stage inwhich they occur andonly finer particles pass to the next succeeding stage for furthercomminution. In this way, extremely fine products are secured.

These and other objects of the invention. will become more apparent fromthe following description, read in conjunction with the accompanyingdrawings, in which:

Figure 1 is a vertical sectional view through a preferred form ofmultiple stage apparatus designed particularly for efiecting finegrinding;

Figure -2 is a vertical section taken on the plane indicated at 22 inFigure 1;

Figure 3 is a sectional plan view taken on the broken surface the traceof which is indicated at 33 in Figure 1; and

Figure 4 is a section taken through the central region of an interstagenozzle, as indicated by the trace 1i4 in Figure 3.

The apparatus which is disclosed comprises a first stage chamber 2,preferably in the form of an oval enclosure having substantially planevertical faces, as indicated in the figures. While this enclosure is notshown as tubular in form, a tubular type of chamber may be provided and,more particularly, one of the type specifically illustrated in myapplication Serial No. 235,139,

.pipes i 6.

referred to above. Whether the central portion of the chamber is or isnot available for flow, nexertheless a centrifugal separation will occuras the fluid carrying material circulates along the oval bounding walls,thereby efiecting return 01' particles to the region of high velocityjets.

The material to be comminuted or dried is inber 2 to the extent or thevelocity head resulting from the recirculation.

The chamber 26, like the chamber 2, is desirably oval in form, and maybe tubular. The jets troduced through the passage 4, being dispersedinto the chamber by means of the jet from a nozzle i, which aspirates itfrom a connection 8 communicating with a supply tank i0, which may beprovided with a suitable valve as indicated at i4 to provide for theassage of material into chamber Ill from a hopper l2 without interferingwith the presure conditions existing in the first stage which may beunder rather high pressure. The pressure in the chamber ID will, ofcourse, be substantially less than that in chamber 2 by reason of theaspirator action in nozzle 6.

Directing elastic fluid into the chamber 2 are a series of nozzles l8receiving elastic fluid from These nozzles are preferably so arrangedthat their jets impinge on each other and are desirably of abrupt typeto secure a maximum of turbulence in the jets to effect the greatestnumber of impacts of the particles on each other. The nozzles are sodirected, as indicated in the figures, that the jets have a component inan approximately tangential direction with respect to the lower curvedwall ofthe chamber 2, thus fluid within the chamber. This recirculationtakes place at high velocity with resultant tendency to throw largerparticles selectively outwardiy, whereby, upon recirculation they passinto the high velocity jets for'regrinding.

Extending from one side wall of the chamber 2 at a level above that ofthe nozzles i8, are nozzles 20, which, it will be noted from Figure 3,which illustrates one of these, bear some resemblance to turbinebuckets. From the arrangement with respect to the direction ofrecirculation in chamber 2, it will be evident that to enter thesenozzles, the elastic fluid and particles carried by it must reversetheir direction of flow. Preferably the nozzles provided at 20 are soshaped interiorly as to provide a-breaking of the fluid away from thewall on the concave sides of these nozzles, whereby there are set upeddies aiding in the production of turbulence and producing still moreeffective grinding by reason of their presence in a regionof intenselyhigh velocity. Into this concave side of the nozzle passage there may beintroduced additional high pressure elastic fluid in relatively smallquantity from a chamber 22 through openings 24, illustrated in Figures 3and 4. It will be noted that the vertical crosssection of these nozzlesat any point is essentially rectangular, resembling thereby turbinebuckets of conventional type or theinterstage nozzles used in turbinepractice. By the introduction of the high pressure and high temperatureelastic fluid in the small auxiliary nozzle openings, the fluid isreheated to some extent between the stages and the velocity of flow isincreased. The design of the nozzles 20 is such that, taking intoaccount the introduction of additional fluid and reheating, the actionis esentially that of abrupt nozzles, thereby projecting into the secondstage chamber 26 jets having intense turbulent flow. It will be notedthat, due to the recirculation in chamber 2, the initial pressure ateach of the nozzles 20 will be less than the static pressure of thechamproviding for a recirculatory movement of the discharged from thenozzles 20 are directed in a general tangential direction into thechamber 26 with the resultant production of high velocity recirculationin this chamber with centrifugal separation of heavier particles ofmaterial.

Assuming thatthe complete expansion may be effected in three stages,athird chamber 30 may constitute the last of the chambers. Into thisthere are projected high velocity jets from nozzles 28, which aredirected from the chamber 28 in a direction opposite the flow therein,so that here again a centrifugal separation takes place at the entranceto the nozzle. The nozzles 28 desirably expand the elastic fluid to thefinal pressure, and are accordingly of a converging-diverging type. Inorder to secure a highly turbulent flow, it is desirable that thesenozzles be of an over-expanded type, rather than of a De Laval type,which would produce a jet relatively ineilective for grinding purposes.In this nozzle, as in the case of nozzle 20, auxiliary high pressureelastic fluid may be introduced through openings 32 to increase thetemperature and,to some extent, the presure to secure very highvelocities. The gas so introduced also substantially increases thevelocity of flow. It will be evident that the nozzles described are ofan illustrative nature and that it is quite possible to utilize otherforms of nozzles as described in my application Serial No. 235.139.

The third stage chamber 30 is also preferably oval in form and, as willbe evident from Figure 2, nozzles 28 are directed from a location abovethe bottom of the chamber 28 tangentially into the lowermost portion ofthe chamber 30. Thus again centrifugal action tends to prevent thepassage of large particles from the chamber 26 into the chamber 30.

From a central portion of the chamber 30 there extends an outlet 34,communicating with the upper portion 36 of a conventional separator 38.From the separator the exhaust gases pass into a pipe 40 and may bepartially released to the atmosphere through a valve controlled passage42, or may be passed through a valve controlled passage 44 to a jacket46 surrounding the three stages of the apparatus and provided with anexhaust outlet 48 and a drain 5|) for condensate in the event that acondensible vapor is being used as the elastic fluid or drying is beingeffected and condensation of the liquid being evaporated may occur.

In the apparatus specifically described, it will be notedthatthroughoutthe grinding stages there are a large number of tendencies forcentrifugal separation to occur. First the location of nozzles 20 withrespect to the bottom of chamber 2 tends to prevent entrance into themof larger particles which recirculate along the walls of chamber 2.Secondly, particles entering the nozzle 20. must undergo an abruptchange of direction by reason of the fashion in which the nozzles 20extend from the chamber wall. Here again occurs a tendency towardthecentrifugal rejection of large particles. Two similar actions againoccur to prevent the entrance of larger particles into the nozzles 28and finally the central position of the tube 34 in which chamber 30tends to prevent passage of large particles from the third stage. Thusonly the finest of particles can reach theseparator and larger particlesare subject to repeated recirculations in the various stages.

As was noted above, the pressure at the entrances to the interstagenozzles will be somewhat less than the static pressures in the chambersdue to a Pitot tube effect at their entrances. They are designed to takethis into account and give at least critical pressure drops to secureacoustic velocities where they discharge into the next chambers. Ifseparation is not so essential, a reversal of flow to enter the nozzlesneed not be provided, but the nozzles may extend from the chambers inthe direction of circulatory flow therethrough. In such case, the fluidwill have an initial velocity of approach tending toward an increase inthe entrance pressure at the nozzle. Of course, if the nozzles havesuiiicient capacity, there is no conversion of the kinetic approachenergy into pressure, but the velocity through the nozzle is increasedto a substantial degree by this velocity of approach. For a givenoverall pressure drop, therefore, more stages may possibly be providedin case such discharge'in the direction of flow is effected. v

The desirability of stages will be evident from consideration of sometypical figures. Assume, for instance, that steam is being used as theelastic fluid at a pressure of 105 pounds per square inch and at atemperature of 420 F. or 88 F. superheat. If an acoustic velocity isacquired in the nozzle, the pressure at the most contracted part of thejet will be 61 pounds per square inch. The resulting superheat will be22 F., and the temperature 316 F. There will be 7.25 cubic feet of steamper pound. The original heat content of the steam will be 1235 B. t. u.sper pound, while the heat content at the pressure of 61 pounds will be1188 B. t. u.s. Thus a difference of 47 B. t. u.s will produce avelocity of 1530 feet per second. The steam will still further expand inthe chamber into which it discharges and at the exhaust in a typicalsingle stage grinder will be approximately at atmospheric pressure,still having, however, a temperature of 300 F. This latter expansion inthe chamber and through the exhaust will not perform any useful work. Ifthe pressure is raised to 175 pounds per square inch, the velocity atthe tip of the nozzle is raised only to 1545 feet per second, so thatthere is very little increase in the kinetic energy of the jet.

By properly selecting the rate of expansion in the nozzles of successivestages of a multiple stage apparatus so as to distribute the totalpressure drop between the stages and to obtain the highest velocities ineach jet thus produced, it is possible to secure the most efiicientgrinding action. For instance, assuming steam at 300 pounds pressureused in the first stage superheated to 800 F., this will give 17 4pounds pressure at the tip of the first nozzles if they are of abrupttype. At the entrance to the second nozzles, the pressure may havedropped to 150 pounds in the mill itself. The velocity attained inabrupt nozzles in the first stage will be about 1850 feet per second. Inthe second stage the same type of nozzles will give 1730 feet per secondif a critical pressure drop occurs. Thus approximately the same grindingeffect can be expected if the second stage nozzles are arranged similarto those of the first stage. At the tips of the second nozzles thepressure will still be 82 pounds per square inch sufficient to providenozzles of a third stage with more than a critical pressure drop.

It may be pointed out that while abrupt nozthe turbulence necessary toinsure proper entrainment of the material, the material leaving thisstage will be already finely ground and entrained in the fiuid, so thatbetween the first and second stages there may be used a type of nozzleadapted to produce quite high velocities but with less turbulence, suchas, for example, an overexpanded nozzle. It may be noted that turbulenceis desirable even if very high velocities are secured to insure amaximum number of impacts of the particles. However, turbulence here isnot absolutely necessary since the particles in the jets will impingeupon other particles recirculating in the chamber into which the jetdischarges. High velocity may therefore be the primary end to beconsidered, with turbulence a secondary matter. There is, therefore,considerable freedom of choice in the design of the interstage nozzles.

As illustrated in the figures, the volumes of the successive chambersmay be increased to take into account the very substantial increase inspecific volume of the elastic fluid, if it is desired that thecirculating velocities should not increase proportionately. Likewise,either larger numbers of nozzles of similar size or similar numbers ofnozzles of largersize are used in the successive stages to takecare ofthe increasing volume of gas to be handled. It will be evident that thesize of the apparatus and pressures and temperatures used, as well asthe relative arrangements of the nozzles, will depend very largely uponthe desired final product. At times, very .fine products are notdesirable, in which case high circulatory ve- I locities and reverseflow of entrance to nozzles should be avoided, so that moderately largeparticles may pass in succession through the stages.

It will be evident from the above that staging effects improvements inboth the thermodynamic efficiency, i. e'., utilization of a much greaterproportion of the original energy to produce high velocities, and in themechanical features of grinding, i. e., the handling of the material,over the operation of single stage apparatus, and the two advantages maybe used independently to some extent. For example, if very fine grindingis not necessary, material may be fed into each of successive stages andseparation of moderately ground material effected in each, in which casethe carrying of some material from one stage to the next may be merelyincidental. In such apparatus, for example, a separator may be providedbetween the stages with suitable escape valve arrangement for removal ofmaterial without loss of pressure. Thus full use is made of the heat andpressure energy with very full conversion of it to kinetic energy.

The mechanical advantage of staging lies principally in the entrainmentof the material to effect proper grinding. Entrainment is best effectedin a turbulent jet from an abrupt nozzle when the material is thrownthereinto by centrifugal action. Less turbulent jets will entrainmaterial thrown into them centrifugally when the particles are large,but will do so relatively less effectively when the particles are small.In the stages subsequent to the first, however, the entrainment iscomplete and hence very high jet velocities may be attained with littleregard for turbulence to impart to the particles high kinetic energiesto secure effective impact with particles of larger size undergoingrecirculation. These very high velocities are particularly secured whenthe nozzles extend in the direction of recirculation from zles ar quiteimportant in the first stage to secure a preceding stage so that a highapproach velocity is provided.

It may be noted that multiple staging also results in possibleutilization of heat resulting from impact in the breaking up ofparticles. The kinetic energy which the particles have prior to impactis in part transformed into heat which raises the temperature of thegas. The resulting additional heat energy is then transformable intokinetic energy in the subsequent nozzles by expansion.

Shock waves may be produced by overexpansion into a region of pressurelower than that normal for the nozzle discharge. .The sudden variationsof pressure on particles in such waves may be utilized to break up byexplosive action porous materials containing evaporable liquid, forexample, wet asbestos, or the like. In such case, entrainment andheating of the material to a high temperature may occur in the firststage in which the pressure remains above that permitting boiling. Inpassing into the second stage, there will occur a pressure drop suchthat boiling may occur at the temperature acquired in the first stage.Desirably, for most violent explosive action, the temperature in thefirst stage should be raised to 'such extent that there is present theheat necessary to provide fully the latent heat of vaporization requiredin the evaporation.

In this same general fashion, a substance may be maintained in liquidphase in one stage and passed into vapor phase in a subsequent stage atlower pressure.

What I claim and desire to protect by Letters Patent is:

1. The method of pulverising material by the use of an elastic fluidcomprising entrainlng the material in a jet of the fluid from a nozzleconstructed and arranged to expand the elastic fluid at least to thecritical pressure with respect to the initial pressure so that the jetacquires at least the acoustic velocity for the pressure and temperatureconditions of the jet to efiect comminution of the material, applyingheat to the fluid from said jet, and causing it to form a jet flowingfrom a second nozzle with a velocity at least the acoustic velocity forthe pressure and temperature conditions of the second jet.

2. An apparatus comprising a pair of upwardly elongated chambers each ofwhich has outer boundary walls having curved intercepts with planesextending in the direction of said elongation, means for introducinginto the first of said chambers material to be pulverized, at least onenozzle arranged to discharge an elastic fluid into the first chamberwith a substantial tangential component of velocity with respect to thecurved outer boundary walls of said first chamber to entrain particlesof the material and effect recirculation thereof along the chamberboundary walls, the curved flow of fluid along said walls serving toeflect centrifugal separation of suspended particles so that lighterparticles predominate over heavier ones in regions inward of said outerboundary walls, means for leading fluid containing suspended material tothe second chamber from a region of the first chamber in whichcentriiugally separated light particles predominate and in a directionat an angle of at least substantially 90 with the direction ofrecirculation in the first chamber, the last named means comprising atleast one nozzle arranged to discharge elastic fluid into the secondchamber with a substantial tangential component of velocity with respectto the outer boundary walls of the second chamber, means for supplyingto the first mentioned nozzle elastic fluid at a pressure exceeding thefinal pressure in the second chamber by substantially more than thecritical pressure drop from the supply pressure, and means for removingthe material from the fluid from the second chamber.

3. An apparatus comprising a pair of upwardly elongated chambers each ofwhich has outer boundary walls having curved intercepts with planesextending in the direction of said elon gation, means for introducinginto the first of said chambers material to be pulverized, at least onenozzle arranged to discharge an elastic fluid into the first chamberwith a substantial tangential component of velocity with respect to thecurved outer boundary walls of said first chamher to entrain particlesof the material and efl'ect recirculation thereof along the chamberboundary walls, the curved flow of fluid along said walls serving toeffect centrifugal separation of suspended particles so that lighterparticles predominate over heavier ones in regions inward of said outerboundary walls, means for leading fluid containing suspended material toth second chamber from a region of the first chamber in whichcentrifugally separated light particles predominate and in a directionat an angle of at least substantially 90 with the direction ofrecirculation in the first chamber, the last named means comprising atleast one nozzle arranged to discharge elastic fluid into the secondchamber with a substantial tangential component of velocity with respectto the outer boundary walls of the second chamber, means for supplyingto the first mentioned nozzle elastic fluid at a Pressure exceeding thefinal pressure in the second chamber by substantially more than thecritical pressure drop from the supply pressure, and

40 means for removing the material from the fluid from the secondchamber, the first mentioned nozzle being of abrupt type and arranged toimpart to the elastic fluid substantial turbulence and a velocityapproximately the acoustic velocity for the pressure and temperatureconditions in the nozzle jet to effect pulverizing of the material byreason of the turbulent conditions within the jet.

4. An apparatus comprising a pair of upwardly elongated chambers each ofwhich has outer boundary walls having curved intercepts with planesextending in the direction of said elongation, means for introducinginto the first of said chambers material to be pulverized, at least onenozzle arranged to discharge an elastic fluid intothe first chamber witha substantial tangential component of velocity with respect to thecurved outer boundary walls of said first chamber to entrain particlesof the material and effect recirculation thereof along the chamberboundary walls, the curved flow of fluid along said walls serving toeffect centrifugal separation of suspended particles so that lighterparticles predominate over heavier ones in regions inward of said outerboundary walls, means for leading fluid containing suspended material tothe second chamber from a region of the first chamber in whichcentrifugally separated light particles predominate and in a directionat an angle of at least substantially With the direction ofrecirculation in the first chamber, the last named means comprising atleast one nozzle having a curved axis in the direction of flowtherethrough and arranged to discharge elastic fluid into the secondchamber with a substantial tangential component of velocity with respectto the outer boundary walls of the second chamber, means for supplyingto the first mentioned nozzle elastic fluid at a pressure exceeding thefinal pressure in the second chamber by substantially more than thecritical pressure drop fromthe supply pressure, and means for removingthe material from the fluid from the second chamber. I

5. An apparatus comprising a pair of upwardly elongated chambers each ofwhich has outer boundary walls having curved intercepts with planesextending in the direction of said elongation, means for introducinginto the first of said chambers material to be pulverized, at least onenozzle arranged to discharge an elastic fluid into the first chamberwith a substantial tangential component of velocity with respect to thecurved outer boundary walls of said first chamber to entrain particlesof the material and eifect recirculation thereof along the chamberboundary walls, the curved flow of fluid along said walls serving toefi'ect centrifugal separation of suspended particles so that lighterparticles predominate over heavier ones in regions inward of said outerboundar walls, means for leading fluid containing suspended material tothe second chamber from a region of the first chamber in whichcentrifugally separated light particles predominate and in a directionat an angle of at least substantially 90 with the direction ofrecirculation in the first chamber, the last named means comprising atleast one nozzle arranged to discharge elastic fluid into the secondchamber with a substantial tangential component of velocity with respectto the outer boundary walls of the second chamber, means for supplyingto the first mentioned nozzle elastic fluid at a pressure exceeding thefinal pressure in the second chamher by substantially more than thecritical pressure drop from the supply pressure, and means for removingthe material from. the fluid from the second chamber, the firstmentioned nozzle being of abrupt type and arranged to impart to berseach of which has curved outer boundary walls, means for introducinginto the first of said chambers material to be pulverized, at least onenozzle arranged to discharge an elastic. fluid into the first chamberwith a substantial tangential component of velocit with respect to thecurved outer boundary walls of said first chamber to entrain particlesof the material and efiect recirculation thereof along the chamberboundary walls, the curved fiow of fluid along said walls serving toeffect centrifugal separation of suspended particles so that lighterparticles predominate over heavier ones in regions inward of said outerboundary walls, means for leading fluid containing suspended material tothe second chamber from a region of the first chamber in whichcentrifugally separated light particles predominate and in a directionat an angle of at least substantially 90 with the direction ofrecirculation in the first chamber, the last named means comprising atleast one nozzle arranged to discharge elastic fluid into the secondchamber with a substantial tangential component of velocity with respectto the outer boundary walls of the second chamber, means for supplyingto the first mentioned nozzle elastic fluid at a pressure exceeding thefinal pressure in the second chamber by substantially more than thecritical pressure drop from the supply pressure, and means for removingthe material from the fluid from the second chamber.

7. The method ofpulverizing material by the use of an elastic fluidworking between predetermined initial and flnal pressures, thedifierence between the initial and final pressures being substantiallygreater than the critical pressure drop from the initial pressure,comprising entraining particles of the material in a turbulent jetproduced by expansion of tm elastic fluid substantially to the criticalpressure and having a velocity approximately the acoustic velocity forthe temperature and pressure conditions of the jet so that pulverizingof the material occurs due to the turbulence of the jet, causing theelastic fluid from the jet carrying entrained particles to follow acurved path to effect centrifugal classification of particles, and thenfurther expanding the elastic fluid carrying centrifugally separatedlighter particles and substantially free of heavier particles to form asecond jet having a velocity at least the acoustic velocity for thetemperature and pressure conditions of the second jet.

8. An apparatus comprising a pair of chambers each of which has curvedouter boundary walls, means for introducing into the first of saidchambers material to be pulverized, at least one nozzle arranged todischarge an elastic fluid into the first chamber with a substantialtangential component of velocity with respect to the curved outerboundary walls of said first chamber to entrain particles of thematerial and effect recirculation thereof along the chamber boundarywalls, the curved flow of fluid along said walls serving to effectcentrifugal separation of suspended particles so that lighter particlespredominate over heavier ones in regions inward of said outer :boundarywalls, means for leading fluid containing suspended material to thesecond chamber from a region of the first chamber in which centrifugallyseparated light particles predominate and in a direction at an angle ofat least substantially 90 with the direction of recirculation in thefirst chamber, the last named means comprising at least one nozzlearranged to discharge elastic fluid into the second chamber with asubstantial tangential component of velocity with respect to the outerboundary walls of the second chamber, means for supplying to the firstmentioned nozzle elastic fluid at a pressure exceeding the finalpressure in the second chamber by substantially more than the criticalpressure drop from the supply pressure, means for feeding auxiliaryelastic fluid to the second nozzle, and

means for removing the material from the fluid from the second chamber.

9. The method of pulverizing material by the use of an elastic fluidworking between predetermined initial and final pressures, thedifference between the initial and final pressures being substantiallygreater than the critical pressure drop from the initial pressure,comprising entraining particles of the material in a turbulent jetproduced by expansion of the elastic fluid substantially t0 the criticalpressure and having a velocity approximately the acoustic velocity forthe temperature and pressure conditions of the jet so that pulverizingof the material occurs'due to the turbulence of the jet, causing theelastic fluid from the jet carrying entrained particles to follow acurved path to effect centrifugal classification of particles, and thenfurther expanding the elastic fluid substantially free of particles oflarge size to form a second jet having a velocity at least the acousticvelocity for the temperature and pressure conditions of the second jetto efi'ect further pulverizing of materials.

10. An apparatus for pulverising material by the use of an elastic fluidworking between predetermined initial and final pressures, thedifference between the initial and final pressures being substantiallygreater than the critical pressure drop from the initial pressure,comprising a primary pulverizing chamber, having curved flow-directingwalls, means for introducing material to be pulverized into the chamber,a nozzle arranged to discharge the elastic fluid into the chamber toentrain particles of the material and carry said particles through asubstantial curved path of flow within said chamber to effectcentrifugal separation of particles of different sizes, means forsupplying the elastic fluid to the nozzle at said initial pressure, thenozzle being conaseasce structed and arranged to expand the elasticfluid substantially to the critical pressure with respect to the initialpressure to impart to the elastic fluid substantial turbulence and avelocity approximately the acoustic velocity fcr the temperature andpressure conditions of the nozzle jet to eflect pulverizing ot thematerial by reason of the turbulent conditions within the jet, the jetbeing directed in the direction of the walls of the apparatus to avoidsubstantial impact of the entrained particles with parts of theapparatus, a subsequent pulverizing chamber, a second nozzle arranged todischarge elastic fluid into the subsequent chamber, the first chamberhaving a region of relatively smooth flow substantially spaced from anyelastic fluid jet and containing predominantly centrifugally separatedlight particles, means for supplying fluid containing suspendedcentrifugally separated, light material solely from said smooth flowregion of the first chamber to said second nozzle, the second nozzlebeing constructed and arranged to expand the elastic fluid further andimpart to it a velocity at least the acoustic velocity for thetemperature and pressure conditions of the second nozzle jet, and meansfor removing the material from the fluid from the subsequent chamber.

NICHOLAS N. STEPHANOFF.

